Total hip replacement surgery involves the introduction of an artificial hip joint in a patient. The artificial hip joint typically consists of a pelvic implant and a femoral implant. The pelvic implant is a cup received in the acetabulum. The femoral implant consists of a spherical portion received at an end of a longitudinal implant portion, or a femoral implant secured to the resurfaced femoral head. In the first case, the longitudinal implant portion is introduced into the intramedullary canal of the resected femur, with the spherical portion being generally centered with respect to the previous position of the femoral head. Therefore, the femoral head (i.e., spherical portion of the femoral implant) and the cup (i.e., pelvic implant) coact to create the artificial hip joint.
Different output values are of concern in hip replacement surgery. In order to reproduce a natural and/or improved gait and range of motion to a patient, the position and orientation of the implants, the medio-lateral offset of the femur and the limb length discrepancy must be considered during surgery. The work of the surgeon during hip replacement surgery will have a direct effect on these output values, and a successful surgery will relieve pain, provide motion with stability and correct deformities.
There is no precise definition of the intraoperative limb length discrepancy (hereinafter “intraop-LLD”) and intraoperative medio-lateral offset (hereinafter “intraop-MLO”). On the preoperative X-rays, surgeons usually measure preoperative limb length discrepancy (hereinafter “preop-LLD”) along the vertical axis of the body as a relation between the interischial line of the pelvis and the lesser trochanter of the femur. Intraoperatively, in order to obtain reasonable measurements that are then possible to validate with X-ray measurements, the surgeons have to align the leg along the vertical axis of the body. This alignment is highly dependent on the surgeon skills and experience. Changes in adduction/abduction of the leg will significantly alter the measurement and introduce measurements errors.
The accuracy of the measurements rests heavily on the surgeon's ability to reposition the leg accurately before each measurement. Therefore, in order to obtain an accurate intraop-LLD and intraop-MLO measurement, the leg, after the implant reduction, must be realigned in the same orientation as before the dislocation. Again, changes in adduction/abduction, flexion/extension and rotation of the leg will significantly alter the measurement.
Failure to provide a robust and accurate method for leg length and offset measurement intraoperatively might lead to the postoperative leg length inequality. This in turn might lead to patient dissatisfaction and/or discomfort, functional impairment (low back pain, static nerve palsy, abductor weakness, dysfunctional gait), unstable hip joint, early mechanical loosening